Figure 7: Illustration of WS-H+ WEG in reservoir of pH 6.9 and pH 12.5, creating a concentration gradient. b) FT-IR of WS, WS-H+ and WS-H+ on NaOH reservoir. c) TGA of Natural WS, Treated WS and WS-H+. d) VOC enhancements of nano-engineered WS-H+. e) ISCC of WS-H+ (16 mm X 18 mm) on DI water. f) ISCC of WS-H+ (16 mm X 18 mm) on alkaline solution.
The WS-H+ samples were separately immersed into neutral reservoir of pH 6.9 as well as the alkaline reservoir with a pH of 12.5. Because of gravity, the top acid solutions travel toward the reservoir, whereas the solutions in the reservoir move upwards because of the action of capillary, causing the formation of concentration gradients as illustrated in Figure 7 (a). Figure 7 (d) confirms the Voc of 960 mV and Figure 7 (e) confirms the maximum Isc of 288 µA when the sample was submerged in neutral solutions. Figure 7 (d) also confirms the Voc of 1.21 V when WS-H+ was immersed in a solution with a pH of 12.5 and the Isc reaches its maximum value of 1 mA as seen in Figure 7 (f) . The difference in pH levels between the WS-H+ surface and the reservoir break apart the surface functional groups unevenly, leading to the enhanced output voltage and current. On top of that, these strong acid treatments enhance the surface area and surface charge.[54,55] When the WS-H+ is placed on the alkaline reservoir, gravity causes the H+ ions to travel across the channel and fall into the reservoir underneath. Consequently, the H+ ions react spontaneously with OH- ions following the equation of H+ + OH- = H2O resulting in depletion of H+ ions. Therefore, continuous migration of H+ occurs in this acid-base reaction process. Furthermore, capillary-assisted upward movement of the OH- ions and gravity-assisted downward migration of H+ ions would result in the formation of a concentration gradient. The non-uniform distribution of a strong acid over the alkaline storage region would lead to an increase in both output voltage and current, as presented in the Figure 7 (d, f). A potential difference is also created between the two electrodes because of the continuous movements of the H+ and OH- ions. Thus, the WS-H+ devices achieved a revolutionary open circuit voltage of 1.21 V and a maximum short circuit current of 1 mA because of the combined influence of acid-base chemical processes, concentration gradients, streaming, and evaporation potentials compared with the original (WS-WEG) physically driven Voc of 620 mV. This substantial contribution from the chemical reactions and concentration gradients were overlooked in the previous evaporation-driven WEG devices.[16,18,56] The porous nutshells can also be considered as a separation membrane between the acid and base solutions. To sustain the reactions and concentration gradients the concentrated acid solutions were introduced on the top surface of the WS-H+ at a rate of 0.1 ml/h. This remarkably efficient hydrovoltaic device demonstrates the highest current density of 347.2 μA/cm2, representing a significant advancement.

Underlying operational mechanism of the device

The mechanism of this water-induced hydrovoltaic electricity generation is relatively complex. The basic mechanism involves ionization of the abundant functional groups on the channel wall of the nutshell surface upon water adsorption, leading to a negative surface charge. Meanwhile, the positive ions of the water will be absorbed on the negatively charged surface of the nutshell (NS), forming an electrical double layer (EDL),[57] illustrated in Figure 8 (b). The micro/nanochannels of the NS structures are considered as passive diffusion channels. In response to concentration gradients, passive diffusion of ions occurs across the shell structure without the need for outward energy input. The porous structures of the NS facilitate the movement of ions along their concentration gradient. Directional water flow inside the channels continues due to the water absorption on one side and evaporation on the other side. Because of the shearing effect of the water flow, the positive charged ions start to accumulate on the evaporation side of the NS. Only the positive ions can effectively traverse the negatively charged micro/nanochannels due to the strong electrostatic repulsion exerted by the negatively charged surface, hindering the penetration of negative ions.[21,56]Due to the separation of charges, potential differences are created between the electrodes that drive electricity generation through the phenomenon of streaming potential. The flow direction determines the positive and negative polarity observed during measurements. An electric field is established as ions accumulate along the flow of water.[21,58] The streaming potential increases linearly with the pressure difference that drives the water flow. The alteration in polarity inside the device is caused by the transpiration of water and the transport of ions via the micro/nano channels. As water drops on the top end, positively charged ions concentrate at the bottom electrode, generating positive polarity in the flow’s direction. When water flow is exclusively dependent on the natural evaporation, the flow reverses from bottom to top, resulting in a polarity inversion that charges the top electrode positive. The streaming potential is contingent upon surface chemistry, ion transport, and flow dynamics within the micro/nano channels. Alterations in flow direction immediately affect the polarity of the generated voltage.
The phenomenon of streaming potential and streaming current can be explained by Equation 1 and Equation 2 and illustrated in Figure 8 (a) . Considering the cross-sectional area A as(πd2)/4 , the vertical pressure differenceΔP as (4γcosθ)/d , the ultimate streaming potential and current can be expressed by Equation 3 and Equation 4 .[59] The magnitude of the streaming potential depends on several factors, including the properties of the liquid, the surface characteristics of the micro/nanochannel, and the flow rate of the liquid as seen Equation 3. Here micro/nanochannels of the NS that transport water are considered as a capillary tube. Water absorption within porous materials is contingent upon capillary forces, which can accelerate or deter water intrusion into the pores.
\(V_{s}=\frac{Ɛ_{0}Ɛ_{r}\mathrm{\Delta}P\zeta}{\text{µσ}}\) Equation 1
\(I_{s}=\frac{AƐ_{0}Ɛ_{r}\mathrm{\Delta}P\zeta}{\text{µL}}\) Equation 2
\(V_{s}=\frac{4\varepsilon_{r}\epsilon_{0}\text{γζcosθ}}{\text{σµd}}\)Equation 3
\(I_{s}=\frac{\text{πγd}\varepsilon_{r}\epsilon_{0}\text{γζcosθ}}{\text{µL}}\)Equation 4
\(R=\frac{8L\mu\ }{\pi r^{4}}\) Equation 5
where L is capillary tube length, A is the cross-sectional area, ζ is the inner surface zeta potential, ΔP external pressure difference, µ is dynamic viscosity, Q is the volumetric flow rate, r is the channel radius,ε0, εr, σ, γ , and μ are the dielectric constant, conductivity, specific tension, and viscosity of the DI water; R is the resistance of the water flow.